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Homework Help: The General Solution of the DE y''+y'=tan(t) ?

  1. Mar 11, 2016 #1
    Hello There, I hope I posted this in the right thread.
    I've been struggling with solving this particular Differential Equation and just couldn't find any way to solve it completely..

    1. The problem statement, all variables and given/known data

    I am only required to find the general solution of the differential equation
    y'' + y' = tan(t)

    2. Relevant equations
    Wronskian of two functions.
    Characteristic Polynomial of a (homogeneous) Differential Equation.

    3. The attempt at a solution
    First I found the solution to the associated DE y''+y'=0 which gave me y1=1 and y2=e-t
    Thus the general solution is y= u1*y1 + u2*y2 where u1 and u2 are two functions of t to be determined.
    Using the method of variation of parameters I ended up with u1 = -ln|cos(t)| + c1 (Note that the Wronskian of y1 & y2 is -e-t )
    However I ended up with u2 = -∫et*tant dt which is obviously unsolvable..
    You can find attached my work and attempts.
    Is there anything wrong? or is there another way around? like just writing the integral as an infinite series?
    Thank you for your help!

    Attached Files:

  2. jcsd
  3. Mar 11, 2016 #2

    Ray Vickson

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    Science Advisor
    Homework Helper

    Lots of problems have solutions that cannot be written in "closed form"; perhaps this is one of them. What I mean is that you can invent a new function ##\Lambda(t) = \int_0^t \tan(s) e^s \, ds## and can then express your answer in terms of ##\Lambda(\cdot)##.
  4. Mar 11, 2016 #3
    Oh so I just leave it as is? Thought there was some extra step I should make...
    If only that y' was a y life would've been much easier haha
    Thank you for your help !
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